1. Field of the Invention
This invention relates to semiconductive compositions which comprise nitrile group-bearing organic semiconductors such as 7,7,8,8-tetracyanoquinodimethane radical ion salts and more particularly, to organic semiconductor compositions which are improved to suppress, to a minimum, toxic gases from generating at the time of thermal decomposition or combustion of nitrile group-bearing organic semiconductors. The invention also relates to a solid electrolytic capacitor utilizing such improved organic semiconductor compositions.
2. Description of the Prior Art
A number of organic semiconductive compounds are known in the art, typical of which are intermolecular compounds or salts comprising, as an acceptor, nitrile group-bearing organic materials such as 7,7,8,8-tetracyanoquinodimethane, dicyanodichloroparaquinone, tetracyanoethylene, tetracyanonaphthoquinone, and the like. For instance, 7,7,8,8-tetracyanoquinodimethane (hereinafter abbreviated as TCNQ) is a highly electrophilic compound or acceptor which has four nitrile groups (--C.tbd.N) in one molecule thereof. The TCNQ compound readily forms intermolecular compounds or salts with a number of compounds or donors having a low ionization potential, thereby obtaining organic semiconductors having low electric resistance. Such organic semiconductors have relatively high thermal stability though low in electric resistance, so that a number of applications to electronic elements have been heretofore proposed. For example, the TCNQ-base semiconductors have been applied as temperature sensors utilizing the temperature change of the resistance, solid electrolytes utilizing the low resistance and electrochemical activity, logic, memory or recording elements utilizing the switching function in resistance of the semiconductor in response to electric field or light, and display elements utilizing the color change in oxidation and reduction reactions.
In recent years, many electric appliances have been now digitalized. Thus, there is a high demand for capacitors of the type which have a low impedance in a high frequency range and are small in size but large in capacitance. TCNQ and analogous compounds are considered to be very promising as a solid electrolyte for solid electrolytic capacitor of the small-size and large-capacitance type.
The organic semiconductors are dissolved in organic solvents or melted on heating, so that it is possible to apply the semiconductor to an oxide film of a capacitor by dipping. This is very advantageous in that a capacitor having a large capacitance and a good high frequency characteristic can be fabricated using the organic semiconductors without impeding the oxide film by thermal decomposition of a salt as will be experienced in known aluminum or tantalum solid electrolytic capacitors.
DOS No. 3,214,355 describes a solid electrolyte consisting of N-n-propyl or N-iso-propylisoquinoline and TCNQ. In this patent application, the TCNQ ion radical salt is melted and impregnated in a convolutely wound aluminum electrolytic capacitor, thereby permitting strong bonding of the TCNQ salt to the oxide film. The resulting aluminum solid electrolytic capacitor has much improved frequency and temperature characteristics partly owing to the high conductivity of the TCNQ salt. In general, TCNQ ion radical salts have higher conductivity and higher anodizability than manganese dioxide or other inorganic oxides, so that solid electrolytic capacitors using such TCNQ salts have better frequency and temperature characteristics than known solid electrolytic capacitors using inorganic oxides and particularly manganese oxide.
Electronic elements utilizing TCNQ do not produce any problems on safety when used under ordinary conditions. However, if the elements are excessively heated, for example, by fire or by passage of overcurrent, there is a great possibility of evolving toxic gases. More particularly, four CN groups exist in the TCNQ molecule (molecular weight: 204). The bonding between C and N in the CN group is so strong that when thermal decomposition takes place, hydrogen cyanide, HCN, having a molecular weight of 27, may evolve. If four CN groups are all converted into HCN, 529 mg of HCN generates per unit gram of the TCNQ molecule. The TCNQ molecule itself is sublimable and thus may be scattered in air prior to decomposition when heated. However, organic semiconductor of TCNQ obtained by reaction with various donors invariably undergo thermal decomposition when allowed to stand at temperatures as high as about 250.degree. C. or over. This thermal decomposition is an exothermic reaction, so that the temperature of an organic semiconductor itself rises at the time of the decomposition. Accordingly, the decomposition temperature reaches a range of from approximately 300.degree. to 500.degree. C. regardless of an initial temperature at which the decomposition takes place. It is known that the above temperature range is an optimum conditions where hydrogen cyanide is most likely to evolve upon burning of nitrogen-containing organic compounds. Where organic semiconductors comprising TCNQ are heated by some origins, it should be taken into account for fabrication of electronic elements using such organic semiconductors that very toxic hydrogen cyanide gas may generate. In fact, it has been confirmed by the gas mass spectroscopy that when quinolinium(TCNQ).sub.2, known as a compound having a low resistance, is thermally decomposed in air, hydrogen cyanide and acetonitrile, CH.sub.3 CN, chiefly evolve. In particular, the quantitative analysis based on the method prescribed in JIS K0109 (i.e. pyridine-pyrazolone spectrophotometry) revealed that 4 wt% of hydrogen cyanide was generated. This value is far much lower than the theoretical amount of 53% and corresponds to a concentration of about 4 ppm when, for example, 100 mg of the TCNQ salt is thermally decomposed in a space of 1 m.sup.3. From the standpoint of the safety standard for working environments, such a value may be within a tolerance limit. In some cases, however, a local concentration may become very high, so that generation of hydrogen cyanide at the time of thermal decomposition of TCNQ salts should be suppressed to a minimum.
It is known that when electrolytic capacitors are inversely connected, a great current flows therethrough and thus the temperature of the electrolyte rises, causing abnormalities such as breakage of the capacitor. We confirmed that when an aluminum electrolytic capacitor using a solid electrolyte of N-n-butylisoquinolinium(TCNQ).sub.2 was applied with a current of 1 A, it was broken down in about 30 seconds while blowing off the decomposed semiconductor. The evolved gas was collected in a one liter glass container and subjected to measurement of HCN, with the result that the amount of the gas was 2,000 ppm.